In the general field of materials testing, a number of different types of material mixtures are amenable to testing by subjecting specimens of the material to compressive forces while concomitantly imposing shear forces on the mixture and then measuring the degree of compaction the specimen undergoes in response to the compression and shearing. As an example, such testing has been found to be useful in measuring asphalt mixture samples in the evaluation of the asphalt mixture used in paving road surfaces as a measure of the quality of the road surface.
An asphalt paving mixture generally comprises a crushed rock aggregate, a bituminous binder, and air voids. Because the individual particles of rock constituting the aggregate are irregular in shape and size, when initially mixed, there are numerous small air voids in the mixture. The strength, durability and cost of asphalt pavement are directly related to the type and size of the particles of rock found in the aggregate, the proportion of binder in the aggregate and the amount of air voids in the final pavement after it has been rolled out, along with other factors. Too much bituminous binder, for instance, under compression fills in substantially all of the air voids and causes flowing of the asphalt mixture in response to the compressive forces which eventually leads to early deterioration of the road surface. Too little bituminous binder can leave the road surface brittle and porous, subjecting the surface to cracking and susceptible to freeze-thaw disruption with the entry of water for roads situated in those climates subjected to freezing temperatures.
The purpose of gyratory compaction testing is to subject a sample of the asphalt mixture to compression and shear forces to determine the degree of compaction achievable which is directly related to the amount of bituminous binder and air voids present in the asphalt mixture. Known test parameters are the density of the aggregate, bituminous binder, and weight of the sample. The degree of compaction is then used to calculate the change in density due to compaction of the sample during the test run. The results are then useful to determine whether a particular asphalt paving mixture will have the strength and durability required for the anticipated traffic conditions on a particular roadway before the asphalt is applied to the roadway.
The United States Department of Transportation Federal Highway Administration Publication No. FHWA-SA-95-003, Background of Superpave Asphalt Mixture Design and Analysis (February 1995) describes a gyratory compaction test for asphalt paving material and the conclusions which may be inferred from gyratory compaction testing. Devices for performing these tests have been developed by Troxler Electronic Laboratories, Inc., Pine Instrument Company and others.
These guidelines call for precise, reproducible compressive forces, angles of gyration and specified temperatures for testing a sample within the mold. It is a combination of the compressive force and angle of gyration that determines the resultant shear force placed on the specimen. Precise control of compression and angle of gyration is critical in obtaining accurate test results that are reliable in predicting the actual conditions of density, alignment of aggregate and appropriate elastic properties within the asphalt mixture. The proper temperature is also critical in maintaining the appropriate viscosity of the sample during the test so as to approach similar to actual conditions during the paving procedure.
In general, the testing protocol requires that a heated and pre-weighed specimen of the asphalt paving mixture be placed in a cylindrical mold. The specimen is then compressed to a predetermined pressure and an initial density calculation is determined. Since the density of the rock and bituminous binder are known, the measured density, in conjunction with the known values of two of the components, is used to calculate the percentage of air voids in the sample. The specimen mold is then moved in a gyratory fashion at a small angle relative to, and around, the long axis of the compressive force as applied to the sample through mold end plates while the sample is kept under a specified amount of compression. The compression force and the gyratory motion of the mold combine to produce a shear stress in the specimen as long as at least one of the end plates remains perpendicular to the longitudinal axis of the compressive force. This gyratory compaction testing is designed to reproduce the shear stresses induced in the asphalt mixture when it is laid down and undergoes vibratory compression from the paving rollers. As noted above, it is the resulting compacted paved asphalt that will determine the quality of the road surface
This shear stress causes individual particles of rock to move, realign and perhaps even to break, thus filling a substantial amount of the voids and reducing the volume of the paving mixture in the mold. The strength, durability, elasticity and thus the suitability of the paving mixture for anticipated traffic conditions is inferred from the reduction in volume, and therefore, change in density of the specimen and other observations made during the test.
The principles of gyratory compaction testing are more fully explained in the above-referenced publication. However, if the gyratory compaction test is to be precise and reliable, the compression force and the angle at which the cylinder mold is inclined as it is gyrated must be held constant within a very narrow range. Generally, maintaining the angle of inclination precisely within the narrow range specified by the testing protocol is far more difficult than maintaining the compressive force within the specified range.
U.S. Pat. No. 2,972,249 issued to McRae, et al., discloses a gyratory compactor device that holds the specimen within a mold chuck between two opposed compression rams. Shear forces are generated by gyrating the mold chuck around the long axis of the compression rams. The mold is gyrated using multiple rollers mounted in a chuck mold oscillator frame, the wheels being offset in relation to each other and engaging a flange on the outside of the chuck mold.
U.S. Pat. No. 5,456,118 issued to Hines, et al., also discloses a gyratory compactor that accomplishes gyration by tilting the mold. This disclosure uses an external mold carriage assembly attached to a rotatable circular base below and a carriage tilt link assembly above. Just as with the McRae device, multiple rollers engage an outside flange rim of the mold and spin around the outside of the mold in a plane tilted to the axis of the compression ram. A secondary consequence of this device is the lateral moment arm of force exerted on the compression ram as the mold and contents are gyrated. Any deflection in the shaft of the compression ram shaft towards the induced tilt axis decreases the amount of shear forces on the sample and changes the testing environment. The amount of lateral moment arm force on the compression ram shaft is not reproducible from test to test. The Hines device also requires the removal of the mold from the tilting assembly to facilitate loading and extracting the test sample. This severely curtails the number of samples that can be run over time because the device requires realignment and recalibration for each run and the mold must be brought back to testing temperature before the next test run.
U.S. Pat. Nos. 4,942,768 and 5,036,709 issued to McRae disclose a gyratory compactor that tests a specimen held in a mold by a chuck. The chuck, not the mold, is then gyrated using a spinning offset roller assembly engaging a flange to the outside of the chuck as the specimen is compressed from the bottom while held in a stationary mold. Gyration of the chuck and its end plate in contact with the specimen effects a kneading action on the specimen within the mold. As with the prior two devices mentioned above, this device uses a bulky assembly, with multiple rollers, spinning to the outside of the specimen mold to accomplish the gyration.
U.S. Pat. No. 5,323,655 issued to Eagan, et al., (Eagan) discloses a gyratory compactor having a cylindrical mold supported by a base assembly, a mechanism within the base assembly for supporting and tilting a bottom of the mold, and compression means at the top of the mold. Eagan discloses that the mold must sit on the tilting mechanism. In addition, Eagan discloses a device wherein the tilting mechanism is built into the device and is not removable, using three rollers engaging the bottom of the mold. The minimum number of rollers needed to accomplish this is three, any less and the mold slides away. To fill or empty the mold, the Eagan device requires that the mold be removed from the device and requires a separate sample extractor be used in order to remove the sample from the mold.
These gyrating assemblies have resulted in testing machines that are large, heavy and therefore not transportable. Their expense substantially influences, and limits, the number of machines available, increasing the likelihood that large mixtures of asphalt will be paved out having never been tested for appropriateness of use. These gyratory inducing mechanisms, using multiple rollers outside the mold against one or more flanges also to the outside, require many moving parts that are held in a large framework and cabinet.
At the conclusion of the gyratory compaction test, the specimen must be stripped from the mold. The Hines and earlier McRae machines discussed above require that the mold be removed from the machine and inserted onto another device where the specimen is pressed out of the mold. These machines must be precisely readjusted every time a sample is run to maintain the angle of inclination because the entire setup has to be taken down and reassembled between test runs. Additional time is lost bringing the mold back up to temperature before the next sample can be run in that mold. While they are suitable for laboratory use, those machines are not readily adaptable for mobile operation and testing samples of paving mix at the point of mixing or at the job site.
Consequently, there is a need for a simple, lightweight gyratory compaction testing machine which controls compression forces and maintains the angle of inclination precisely with minimal adjustment and which may be readily mounted on a van, truck or other vehicle and operated at the site where the asphalt is mixed and/or the roadway is being paved. There is a need for a gyratory testing machine which can strip the specimen from the mold without removing the mold from the gyratory compaction testing machine.